In transcranial magnetic stimulation, a very brief electrical current is passed through an insulated coil wire placed on the scalp. These currents stimulate the cortex (outer part of the brain). They may cause muscle, hand, or arm twitching if the coil is near the part of the brain that controls movement, or they may affect other reflexes or movements. During the study, subjects may be asked to make movements, do simple tasks or tense muscles. To record the electrical activity of muscles, electrodes will be taped to the skin over the muscles tested. In some cases, the studies will be videotaped.